H e a t in co u r tya r d s : a valid a t e d a n d c alib r a t e d p a r a m e t ric s t u dy of h e a t mi tig a tion s t r a t e gie s for
u r b a n cou r tya r d s in t h e N e t h e rl a n d s
Taleg h a ni, M, Tenpie rik, M, va n d e n Dob b els t e e n, A a n d S ailor, D
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Tit l e H e a t in cou r tya r d s : a valida t e d a n d c alib r a t e d p a r a m e t ric s t u dy of h e a t mi tig a tion s t r a t e gi es for u r b a n cou r tya r ds in t h e N e t h e rl a n d s
Aut h or s Taleg h a ni, M, Tenpie rik, M, van d e n Dob b els t e e n , A a n d S ailor, D
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Heat in courtyards:
A validated and calibrated parametric study of heat mitigation
strategies for urban courtyards in the Netherlands
Mohammad Taleghani*1 , Martin Tenpierik1, Andy van den Dobbelsteen1, David J. Sailor2
1 Faculty of Architecture and the Built Environment, Delft University of Technology, Delft, the
Netherlands
2 Department of Mechanical and Materials Engineering, Portland State University, Portland, OR, USA
Abstract
Outdoor thermal comfort in urban spaces is an important contributor to pedestrians’ health. A
parametric study into different geometries and orientations of urban courtyard blocks in the
Netherlands was therefore conducted for the hottest day in the Dutch reference year (19th
June 2000 with the maximum 33°C air temperature). The study also considered the most
severe climate scenario for the Netherlands for the year 2050. Three urban heat mitigation
strategies that moderate the microclimate of the courtyards were investigated: changing the
albedo of the facades of the urban blocks, including water ponds and including urban
vegetation. The results showed that a north-south canyon orientation provides the shortest
and the east-west direction the longest duration of direct sun at the centre of the courtyards.
Moreover, increasing the albedo of the facades actually increased the mean radiant
temperature in a closed urban layout such as a courtyard. In contrast, using a water pool and
urban vegetation cooled the microclimates; providing further evidence of their promise as
strategies for cooling cities. The results are validated through a field measurement and
calibration.
Key words
Urban courtyard blocks, climate change, urban microclimate, heat island mitigation
strategies.
1 Corresponding Author: Mohammad Taleghani [email protected] ; [email protected]. Co-authors contacts: Martin Tenpierik: [email protected] Andy van den Dobbelsteen: [email protected] David J. Sailor: [email protected]
1. Introduction
Growing urbanisation and the extensive consumption of fossil fuels have a profound impact
on the thermal environment in cities. The relatively low reflectivity of urban surfaces
combined with high density of construction in cities results in an accumulation of heat in the
urban environment. The general lack of green (vegetated) areas and surface water also
makes cities warmer. As a result, the cooling demand of urban residents increases [1, 2] and
the heat stress on pedestrians rises [3, 4]. Promising mitigation strategies have been
developed in order to cool urban spaces. These strategies are mainly related to the
configuration of the built environment in accordance with (un)favourable solar radiation,
construction materials used, and presence of water and urban vegetation [5-9].
The novelty of this paper is its focus on heat mitigation strategies in urban courtyard blocks in
the Netherlands as one of the countries prone to climate change, i.e. becoming warmer and
wetter [10, 11]. Thermal studies of urban courtyard designs are mainly studied in hot and arid
environments and less in moderate Western Europe while there are several examples of the
presence of this urban form in different Dutch cities (Figure 1). This paper begins with a
comprehensive review of strategies for cooling a microclimate. It then explores the
application of these strategies to urban courtyards in the Netherlands. Courtyard blocks or
building clusters are commonly used urban patterns in the Netherlands. In phase 1, the
impact of different courtyard geometries and orientations are analysed; in the next phases,
the courtyard models are studied in the context of the Dutch climate in 2050. Finally, three
heat mitigation strategies are studied parametrically for the current situation on the following
features: changing the albedo of the facades of the urban blocks, adding urban green, and
adding water ponds.
Figure 1: Urban courtyard blocks in Amsterdam, Rotterdam and The Hague (left to right).
2. Background
The following brief literature review considers studies of a) urban geometry and courtyards,
b) microclimate design to cope with climate change, c) the effects of albedo on a
microclimate, d) the effects of water on a microclimate, and e) the effects of vegetation on a
microclimate.
a) Urban geometry and courtyards
The interactions between urban geometry and surface properties under a specific climate
generate microclimates. These interactions were first discussed by Olgyay [12] and Oke [3].
Givoni [13] discussed the thermal impact of urban typologies in different climates and arrived
at general design guidelines. He writes that architectural forms, surface materials and urban
morphology (compactness, elongations, etc.) can affect the microclimate environment. On
this topic, courtyard blocks were studied in several climates addressing different benefits. A
comprehensive study on urban courtyards at a latitude of 26-34°N was done by Yezioro,
Capeluto and Shaviv [14] using the SHADING program. They showed that, for cooling
purposes, the best direction of a rectangular courtyard was North-South (NS, i.e. with the
longer facades on East and West), followed by NW-SE, NE-SW, EW (in this order). They
found that the NS direction had the shortest duration of direct sun light in the centre of the
courtyard. This finding is in accordance with climates (or seasons) in which less sun is
desirable. They also investigated summer thermal comfort, and showed that, although the air
temperature difference between shaded and unshaded areas was only 0.5 K, the mean
radiant temperature was different up to 30 C [15]. Steemers, Baker, Crowther, Dubiel,
Nikolopoulou and Ratti [16] proposed six archetypical generic urban forms for London and
compared the incident solar radiation, built potential and daylight admission. They concluded
that large courtyards are environmentally adequate in cold climates, where under certain
geometrical conditions they can act as sun concentrators and retain their sheltering effect
against cold winds. Herrmann and Matzarakis [17] simulated urban courtyards with different
orientations in Freiburg, Germany. They showed that the mean radiant temperature (Tmrt)
was highest for NS and lowest for the EW orientation at midday and at night. During the
night, the mean radiant temperatures were very similar, but the orientation of the courtyard
affects the time of the first increase of Tmrt in the morning, due to direct sun. In the
Netherlands (on average at 52°N), few studies have addressed the Physiological Equivalent
Temperature (PET) or other outdoor thermal comfort indices. Among these, van Esch,
Looman and de Bruin-Hordijk [18] compared urban canyons with street widths of 10, 15, 20
and 25 meters, and EW and NS directions. They concluded that the EW canyons did not
receive sun on the 21st of December, while during summer and in the morning and afternoon,
canyons had direct sun; at noon the sun was blocked. On the shortest day, the NS canyons
got some sun for a short period (even the narrowest canyon) and were fully exposed to the
sun in the mornings and afternoons.
b) Microclimate design to cope with climate change
Global warming is likely to have a significant effect on cooling and heating loads in buildings.
IPCC [19] provides (and updates) estimations for future climates through different climate
scenarios. Accordingly, building and urban designers try to reduce cooling loads of buildings
to cope with climate change in the future. The strategies that they use are mainly through:
1. reducing solar heat loads, for instance with appropriate design, sun shading and
reflecting materials [20-22];
2. using natural cooling (provided by greenery, water and natural ventilation at night) [23,
24]; and
3. using thermal mass in order to stabilise indoor temperatures [25, 26].
Vegetation can simultaneously block and reflect the sun and cool the environment through
evapotranspiration. However, there are few studies addressing the effects of greenery in the
context of future climate scenarios in the Netherlands. Outdoor thermal comfort needs to be
studied to clarify how increasing urban greenery can provide a more comfortable
environment in a warmer future.
c) The effects of albedo on the microclimate
The albedo of a surface or material is defined as the fraction of incident solar radiation that is
reflected [27]. High albedo materials therefore lead to lower surface temperatures, and a
cooler ambient temperature through the mechanism of convection [28]. However,
conventional materials used in urban environments such as asphalt, brick and stone
pavements generally have low albedos (0.05, 0.2 and 0.4 respectively) [29, 30]. The use of
these materials intensifies the urban heat island phenomenon. Several studies have reported
that use of materials with low albedo and high specific heat capacity usually causes a larger
temperature difference between the city and the countryside at night than during the day [31,
32].
In this regard, Doulos, Santamouris and Livada [33] compared 93 commonly used materials
for outdoor pavements. They found that albedo depends on the visible colour, surface texture
(roughness) and the type of material of a pavement. They concluded that smooth, flat and
light tiles made of marble, mosaic and stone had higher albedo than concrete and granite.
Conversely, although a higher albedo results in a lower surface temperature and
consequently cooler indoor environment, it could have negative consequences on the
physical and mental health of pedestrians [34-36]. In the subtropical climate of Shanghai
Yang, Lau and Qian [37] showed that by increasing the ground surface albedo by 0.4, overall
outdoor thermal comfort decreased as reflected by an increase in physiological equivalent
temperature (PET) by 5–7°C. In this way, in a dense urban area with a hot climate, the
albedo of vertical surfaces such as facades will play an important role for the pedestrian’s
thermal (and visual) comfort. In an extreme situation in Tokyo, the use of high albedo
materials on the exterior opposite walls led to a higher indoor cooling demand because of the
increased solar radiation reflected indoors through the windows [38]. Taleghani, Sailor,
Tenpierik and van den Dobbelsteen [39] showed in the temperate climate of Portland (OR,
USA) in a measurement showed a white material (with albedo 0.91) increased the globe and
mean radiant temperature (0.9°C and 2.9°C respectively) while producing a cooler local air
temperature (1.3°C) in comparison with a black pavement (with albedo 0.37). To sum up, the
effect of albedo on both the indoor and the outdoor thermal environment can only be
determined when studied in more detail for each urban situation.
d) The effects of water on the microclimate
The cooling effect of ponds and canals on microclimates has been demonstrated in multiple
studies [40, 41]. In the hot and dry climate of Bornos (Spain), for example, Reynolds and
Carrasco [42] found summer temperature variations inside a courtyard with an enclosed
pond from 26 to 29.5°C while the ambient temperature outside the courtyard varied between
22 and 44°C. Nakayama and Fujita [43] developed a water-holding pavement (consisting of
porous asphalt and a water-holding filler) to increase water presence in urban spaces of
Japan. They reported that the air temperature (Ta) above the water-holding pavement (when
saturated) was 1-2°C lower than above the lawn and 3-5°C lower than above conventional
pavements. In the hot and arid climate of Bahrain Radhi, Fikry and Sharples [44] showed that
lack of water in an urban space could cause a 2-3°C temperature increase in the city and a
3-5°C temperature increase on artificial islands. In addition, through an optimisation study for
the thermal comfort of an urban square in France, Robitu, Musy, Inard and Groleau [45]
reported that the presence of trees and water ponds reduced the mean radiant temperature
by 35-40°C at 1.5 m above the ground.
e) The effects of vegetation on the microclimate
Vegetation has been studied in urban climates [46], mostly in regard to the urban heat island
effect (first studied by Luke Howard in the early 19th century [47]). In contrast to the urban
heat island, the park cool island can reduce the air temperature up to 3-4°C in summer [46,
48-50]. Vegetation cools the environment through two mechanisms [51]:
1. With a higher albedo compared to common pavements as asphalt and brick. Vegetation
reflects more solar radiation [52]; moreover, with a lower specific heat capacity, green
areas accumulate less heat [49, 53].
2. By evapotranspiration, which is the sum of evaporation (from the earth's surface) and
transpiration (from vegetation). The ambient air is cooled by this phenomenon [3, 54, 55].
The evapotranspiration process requires a significant amount of energy from the
microclimate. As noted by Montgomery [56] the latent heat of vaporisation of water is 2324
kJ/kg. Moffat and Schiller [57] found that latent heat transfer from wet grass can result in an
air temperature 6–8°C cooler compared to a similar area with exposed soil. They also found
that 1 m2 of grass absorbs 12 MJ of heat on a sunny day.
The other advantage of green areas is their effect on the energy use for maintaining
comfortable indoor environments. According to Akbari, Division, Laboratory and Energy [58],
Wong, Tay, Wong, Ong and Sia [59] and Carter and Keeler [60] trees and shrubs planted
next to a building can reduce air conditioning costs by 15-35 % (and by 10 % of annual
cooling demand). Likewise, the exposed surface of a black roof with a very low albedo can
be as much as 50°C hotter than the roof surface under a vegetated green roof in summer
[61].
While trees have the advantage to block the sun reaching the ground surface cooling the
entire air space under their canopy, grass reduces the temperature mainly near ground level
[62, 63]. Air temperature reductions due to vegetation are reported as: Miami 16°C, Tokyo
20°C, Singapore 5°C and Athens 8°C [64-67]. The potential cooling benefits of vegetation
are increasingly being exploited in rooftop applications. Vegetated or “green” roofs have
multiple benefits for the urban environment, including a reduction in storm water runoff,
cooling of the urban climate system, and reduction in summer time heat transfer into
buildings [6, 68, 69].
3. Methodology
The study presented in this paper consisted of five phases. In the first phase, 18 courtyard
models were simulated using the ENVI-met software package for 19th June 2000, the hottest
reference day in the Netherlands. These courtyards are given four directions: E-W, N-S, NE-
SW and, NW-SE (see Figure 2, respectively row a, b, c and d). The dimensions of the
courtyards inside the urban blocks vary between 10*10 m2 and 10*50 m2 with steps of 10 m.
ENVI-met is also needed to be validated and calibrated for the climate of the Netherlands.
This process is extensively explained in Section 3.3.
Figure 2: Overview of the basic models for the parametric study, E-W (1st row), N-S (2nd row), SW-NE
(3rd row) and NW-SE (4th row). The reference models used in phases 2 to 5 are highlighted in grey.
The dimensions are for the size of the courtyards, and the buildings have a depth of 9 m.
In Phase 2, the effect of climate change in 2050 was studied. Three models from the
previous phase, 10*10 m2, 10*50 m2 E-W (from row a in Figure 2), and 50*10 m2 N-S (from
row b), were selected as reference models. The other models in-between the mentioned
ones were not simulated, because the first phase showed that the thermal behaviour of these
intermediate models follows a regular pattern, and the three selected are the extreme models
(in size and thermal impact). The weather data used for the simulation of 2050 is explained in
section 3.2.
In the 3rd phase, the effect of changing the albedo of the facades of the urban blocks was
studied, again considering the three reference models. Specifically, the brick surfaces of the
original model (with an albedo of 0.10) were replaced with white marble (0.55) and white
plaster (0.93) [70] to check its effect on the microclimate.
In the 4th phase, the cooling effect of small bodies of water was tested through embedding a
water pool inside the three reference models. The size of the pool was chosen such that in
all models 65% of the ground is allocated to a water pool while the rest is still pavement.
In the 5th phase, the cooling effect of vegetation was addressed. The ground and the roof of
the courtyard blocks were covered with grass.
In every phase of this study, the results of the reference models were compared with the
results of phase 2 (Figure 3).
Figure 3: The research method of the paper. First, the simulation software is validated through field
measurement and calibration (left). Second, a comprehensive parametric study with simulation is done
(right). In the first phase of the parametric study, 18 courtyard models are simulated in four directions.
In the next phases, three reference models which are highlighted in Figure 2 are used for optimisation.
3.1. Simulations
For the study discussed in this paper, the hottest day of the Dutch reference year [71] was
selected for the simulations (19 June 2000). This extreme day was selected to check the
potential of the different courtyards in providing a comfortable microclimate in summer. All
simulations were conducted using the urban computational fluid dynamics software ENVI-
met 3.1 [72]. This program is a three-dimensional microclimate model designed to simulate
the surface, plant and air interactions in an urban environment with a typical resolution of 0.5
to 10 meters in space and 10 second in time. ENVI-met can calculate the air temperature
(°C), vapour pressure (hPa), relative humidity (%), wind velocity (m/s) and mean radiant
temperature (°C) of the centre of models [73]. This program has been extensively validated
and used widely for studying the effect of climate change [74, 75] and the impact of natural
elements on a microclimate [73, 76, 77]. Table 1 shows the simulation conditions used for
the first phase of this study.
Simulation day 19.06.2000
Simulation period 21 hours (04:00-01:00)
Spatial resolution 1m horizontally, 2m vertically
Initial air temperature 19°C
Wind speed 3.5 m/s
Wind direction (N=0, E=90) 187°
Relative humidity (in 2m) 59 %
Cloud coverage 0 Octa (clear sky)
Indoor temperature 20°C
Thermal conductance 0.31 W/(m2K) (walls), 0.33 W/(m2K) (roofs)
Albedo 0.10 (walls), 0.05 (roofs)
Table 1: The conditions used in the basic simulations (phase one of the parametric study).
3.2. Climatic data
The climate of De Bilt (52°N, 4°E), is fairly typical of the Netherlands, and is classified as a
temperate climate zone based on the climatic classification of Köppen-Geiger [78]. The wind
is omnidirectional but South-West is prevailing. The mean annual dry bulb temperature is
10.5°C. For this paper, the reference weather data of De Bilt was used for the simulations
and calculations according to Dutch NEN-5060 standard [71]. According to this standard,
every month of the reference year is represented by a month from a specific year which is
considered representative of the period from 1986 until 2005. The process for developing this
reference year is very similar to the approach for developing Typical Meteorological Year
data [79].
Regarding the future climate scenario in 2050, The Royal Dutch Meteorological Institute
(KNMI) has translated the IPCC variants to four main scenarios in the near future in 2050,
divided as in a matrix of two times two: a moderate and warm scenario (+1°C and +2°C
temperature increase respectively) versus unchanged or changed air circulation patterns: G
(moderate and unchanged air circulation), G+ (moderate and changed air circulation), W
(warm and unchanged air circulation), W+ (warm and changed air circulation). Recent
insights indicate a greater probability towards W and W+ rather than G and G+, implying
higher temperatures throughout the year as well as dryer summers and wetter winters. Table
2 presents an overview of climate characteristics for each of the four climate scenarios.
2050 G G+ W W+
Circulation change No Yes No Yes
Global temperature rise +1°C +1°C +2°C +2°C
Change in air circulation patterns No Yes No Yes
Winter Average temperature +0.9°C +1.1°C +1.8°C +2.3°C
Coldest winter day per year +1.0°C +1.5°C +2.1°C +2.9°C
Average precipitation amount +4% +7% +7% +14%
Number of wet days (≥0.1 mm) 0% +1% 0% +2%
Maximum average daily wind
speed per year
0% +2% -1% +4%
Summer Average temperature +0.9°C +1.4°C +1.7°C +2.8°C
Warmest summer day per year +1.0°C +1.9°C +2.1°C +3.8°C
Average precipitation amount +3% -10% +6% -19%
Number of wet days (≥0.1 mm) -2% -10% -3% -19%
Potential evaporation +3% +8% +7% +15%
Table 2: climate change scenarios for 2050 in the Netherlands [80].
In this paper, the W+ scenario was selected as it is the most extreme scenario for 2050 in
comparison with the current climate. Taleghani, Tenpierik and van den Dobbelsteen [81]
explain how weather data for the year 2050 is constructed from these KNMI climate
scenarios. In the weather file for the year 2050, solar radiation intensity has not changed
since it mainly depends on latitude. However, cloud coverage and precipitation could not be
changed as compared to the current climate because detailed data for the climate scenarios
is lacking. The weather file for the year 2050 thus only differs from the weather file for the
current climate concerning air temperature.
3.3. Validation of ENVI-met
3.3.1 Measurement versus simulation
In this research, one ENVI-met model was validated for the Netherlands through a
comparison between field measurements and simulation results. The measurements were
done within a courtyard building on the campus of Delft University of Technology. A wireless
Vantage Pro2 weather station was used to measure among others drybulb air temperature
with an interval of 5 minutes (Figure 4-a). The sensor of air temperature was protected by a
white shield to minimise the effect of radiation. The height of the data logger is 2 m. The
courtyard environment was measured for 16 days in September 2013. Two random days,
September 22nd and 25th were selected for ENVI-met simulation. The weather data for the
simulation were taken from a weather station located 300 meters from the courtyard. The
data from simulations and measurements are compared in Figure 5 to show the accuracy of
the simulation results. Moreover, the simulation input data are presented in Table 3.
First day Second day
Simulation day 22.09.2013 25.09.2013
Simulation period 28 hours 28 hours
Spatial resolution 3m horizontally, 2m
vertically
3m horizontally, 2m
vertically
Initial air temperature 15.6°C 14°C
Wind speed 1.0 m/s 1.1 m/s
Wind direction (N=0, E=90) 245° 180°
Relative humidity (in 2m) 94 % 87 %
Indoor temperature 20°C 20°C
Thermal conductance 0.31 W/(m2K) (walls),
0.33 W/(m2K) (roofs)
0.31 W/(m2K) (walls),
0.33 W/(m2K) (roofs)
Albedo 0.10 (walls), 0.05 (roofs) 0.10 (walls), 0.05 (roofs)
Table 3: The conditions used in the validation simulations.
Figure 4: a) The weather station (Vantage Pro2) used for measurement in situ, b) the aerial photo of
the measured courtyard, and c) the courtyard model and its surroundings in ENVI-met. The red line
specifies the location of the weather station in the field and the receptor point in the computer model.
The measured air temperatures between 21st and 26th of September are shown in Figure 5
with the black line. The two simulated days are drawn with the grey line (on 22nd and 25th).
On the first day (22nd), the patterns of air temperature between measurement and simulation
are more or less the same. On the second day (25th), the peaks of the hottest hour are
different in number and in time. On the first day, the peak of Ta according to the simulation is
0.5°C higher than according to the measurement. On the second day, the peak of Ta
according to the measurement is 1.2°C higher than according to the simulation. The root
mean square deviation of the dry bulb temperature between simulation and measurement on
the first day is 0.7°C and on the second day is 1.3°C. In Figure 5- right, the total data of
simulation and measurement in the two days are compared in one diagram. The correlation
coefficient between the two sets of data is 0.80.
Figure 5: Comparison of the simulation results (on 22nd and 25th) with the measurements between 21st
and 26th of September (left). The compared two day data are also illustrated in a scattered
graph (right).
3.3.2 Calibration of the ENVI-met simulations
To check the accuracy of the ENVI-met models, the reference models highlighted in Figure 2
are modelled with two different grid sizes (180*180 m2 and 90*90 m2). As it is shown in
Figure 6-a, a courtyard model (10*50 m2 EW) with 8 similar blocks in its surrounding is
modelled in the 180*180 m2 grid size. Then, the same courtyard model is simulated also in
the 90*90 m2 grid size withought neighbouring blocks (Figure 6-b). If the results of the
reference models in the context of these two different grid sizes are identical, further
simulations could be done with the smaller grid size (90*90 m2) to reduce the simulation time.
For this calibration, the air temperature and mean radiant temperature within the courtyards
are compared. The simulations are done under the conditions mentioned in Table 1. Figure
6-c shows the air temperature for the two grid sizes, and Figure 6-d shows both results as
function of each other. Since the air temperatures in the two models do not exactly match,
the trendline line in Figure 6-d is not perfectly 45°. This shows that there is a deviation
between the two situations. In fact, the root mean square deviation of the two situations is
0.31°C. In Figure 6-e and 6-f, this comparison is done for the mean radiant temperature, and
the root mean square deviation in this case is 0.74°C. This shows that mean radiant
temperature is more deviated than air temperature between the two simulations.
Figure 6: a) the courtyard model 10*50 m2 EW in 180*180 grid with similar neighbouring blocks, b) the
same courtyard model withought neighbours and in 90*90 grid size, c) air temperature in different grid
sizes, d) the comparison of the air temperatures in a scattered graph, e) mean radiant temperature in
different grid sizes, and f) the comparison of the mean radiant temperatures in a scattered graph.
This calibration procedure was repeated for the other reference models, 10*10 m2 and 50*10
m2 NS. The results are explained in Table 4. The average root mean square deviations for
air temperature and mean radiant temperature in the reference models are 0.26°C and
0.98°C, respectively. This shows that further simulations with a 90*90 m2 grid only, thus
withought similar urban courtyard blocks, introduces a small but acceptable deviation in air
and mean radiant temperature.
RMSD* for Ta RMSD for Tmrt
10*10 m2 0.32 1.06
10*50 m2 EW 0.31 0.74
50*10 m2 NS 0.15 1.15
Average 0.26 0.98
Table 4: The calibration data of models with two different grid sizes. *RMSD= root mean square
deviation.
4. Results
4.1. Phase 1: Reference study
In this step of the study, 18 urban blocks were simulated for 21 hours on the 29th of June
2000 (the hottest reference day in the Netherlands). The models vary in length and width
from 10 to 50 m with steps of 10 m; and have four main orientations N-S, E-W, NW-SE, and
NE-SW. The solar radiation reaching each courtyard was illustrated graphically, and also the
mean radiant temperature of the receptor centred in each courtyard was described in this
phase.
4.1.1. Solar radiation
The first phase commenced with a solar radiation analysis for a point at the centre of each
courtyard at 1.2 m height. On the summer day investigated, the sun rises at 05:18 h and sets
at 22:03 h. Figure 7 illustrates the duration of direct solar radiation on the central point of
each courtyard model. In the first row (a), the courtyards are directed E-W. For this
orientation, the duration of direct solar radiation increases from 4 hours and 32 minutes for
the 10*10 m2 courtyard to 11 hours and 44 minutes for the 10*50 m2 E-W courtyard.
Moreover, the first courtyard receives direct sun from 10:03 h till 14:35 h; the widest
courtyard from 06:27 h till 18:11 h.
Regarding the second row (b), the courtyards are extended in N-S direction. In contrast to
the previous urban blocks, the duration of direct solar radiation does not change if the size of
the courtyard increases from 10*10 m2 to 10*50 m2; it is always 4 hours and 32 minutes. This
shows that the east and west parts of the urban block are the main barriers against solar
radiation.
In the third row (c), the urban blocks are rotated 45˚ towards NE-SW direction. Here, the
courtyards are also extended from 10*10 m2 to 10*50 m2. The first rotated courtyard (10*10
m2) receives direct sun at midday for 3 hours and 2 minutes. For the remainder of the
models, this duration is 5 hours and 13 minutes starting at 10:48 h and ending at 16:01 h
(mainly in the afternoon rather than in the morning).
In contrast, the last row (d) with NW-SE orientation has the same duration of direct solar
radiation but between 08:37 h and 13:50 h. Insolation for this orientation happens mainly in
the morning rather than in the afternoon. The duration of direct sun is described in Table 6
(Appendix).
Figure 7: The sun rays of the models on 19th of June. The grey regions show the period that direct sun
light reaches the centre of the courtyards (between the first and last rays of sun). The Figure is
produced by Sketchup (Chronoloux plugin). The data are taken at 1.60 meter height.
Figure 8: Air temperature distribution of the urban block models at 16:00 h (time of peak temperature),
on the 19th of June. The data are taken at 1.60 meter height.
4.1.2. Mean radiant temperature (Tmrt)
The mean radiant temperature, Tmrt, is defined as “the uniform temperature of an imaginary
enclosure in which the radiant heat transfer from the human body is equal to the radiant heat
transfer in the actual non-uniform enclosure” [82]. It is considered as a means of expressing
the influence of radiation from surfaces and of solar radiation on human thermal comfort. In
the outdoor environment, direct solar radiation plays the most important role. Figure 8
presents the mean radiant temperature of the central points inside the courtyards on the
simulated day as calculated by ENVI-met.
In Figure 9a, corresponding to the urban blocks with E-W orientation (a), the mean radiant
temperature suddenly rises when the sun irradiates the central point. In the 10*10 m2
courtyard, this is exactly between 10:03 h and 14:35 h. This duration fits with Figure 7. This
duration increases for the other models, all in correspondence to Figure 7. However, during
midday a decrease in mean radiant temperature occurs for the last four models. During these
hours the sun is very close to the southern façade of the courtyards.
In Figure 9b, corresponding to the urban blocks with N-S orientation (b), the times where Tmrt
increases is in accordance with the times of direct solar irradiation. In this series of models,
the duration of direct sun is the same for all models (4 h and 43 min). However, the
differences between the 10*10 m2 and the four other models is related to the southern part of
the urban block. In the 10*10 m2 model, the sunrays tip the top of the southern façade
around noon as a result of which only scattered sunrays reach the centre point of the
courtyard. However, by increasing the size of the courtyard to 20*10 m2 N-S and beyond, the
centre point gets full solar exposure around noon.To add up, Tmrt in these bigger urban blocks
rises equally.
Considering Figures 8c and 8d, corresponding to the NE-SW and NW-SE orientations
respectively (c and d), Tmrt rises and decreases in corresponce to direct solar exposure of the
centre point. In this way, the first row of the rotated courtyards (NE-SW) receives direct sun
mostly in the afternoon, and the fourth row of the courtyards (NW-SE) receives sun in the
early morning. Consequently, Tmrt increases from noon till afternoon, and from early morning
till early afternoon, respectively.
Figure 9: Mean radiant temperature at the height of 1.60 m at the centre of all urban blocks (a) to (d)
with the same order as in Figure 7. Ro means that it corresponds to a rotated courtyard.
4.2. Phase 2: the climate of 2050
In the second phase of this study, the three reference courtyard models were considered for
the severest climate scenario for the Netherlands in 2050 (W+). As an illustration, two profile
sections from the 10*50 m2-EW model are shown in figure 10 in the current and future
climate scenario. These sections are at 16:00 (the hottest hour in the reference year) and
considering the orientation of the sun, the east parts of the courtyards are warmer than their
west. In the current climate, the hottest temperature close to the ground is 23°C (296 K),
while it is above 25°C in 2050.
Figure 11-a compares Tmrt of the reference models in 2050 (grey lines) to the current climate
(black lines). As explained in the section on methodology, the weather data for the year 2050
only differs from the weather data of the current climate in air temperature. Solar radiation
duration and intensity are identical in both data sets. According to ISO7726 [82], Equation 1
shows how Ta has a small effect on Tmrt:
𝑇𝑚𝑟𝑡 = [(𝐺𝑇 + 273)4 +1.1×108×𝜈𝑎
0.6
ɛ×𝐷0.4(𝐺𝑇 − 𝑇𝑎)] 0.25 − 273 (1)
Where Tmrt is the mean radiant temperature (C), GT is the globe temperature (C),
𝜈𝑎 is the air velocity near the globe (m/s), ɛ is the emissivity of the globe which normally is
assumed 0.95, D is the diameter of the globe (m) which typically is 0.15 m, and Ta is the air
temperature (C).
Since the effect of Ta on Tmrt is very low, Tmrt is only slightly higher in 2050 than in the current
climate. On the other hand, Ta increases by 3°C in the 2050 scenario. These results are
more or less similar for all three reference models.
Figure 10: Comparison of air temperature (potential temperature) of the 10*50 m2 EW model in the
current climate and in 2050 (on 19 June at 16:00).
Figure 11: Mean radiant temperature of reference models in comparison with: a) the 2050 W+ climate scenario; b) higher albedo of plaster; c) courtyards with a water pool; d) courtyards with a green area.
Based on these differences between the current and the future climate in the Netherlands,
the next three phases in the analysis investigated possible heat mitigation strategies.
4.3. Phase 3: The albedo effect
In the third phase of this study, the albedo of the models’ facades was increased. This
change allowed us to understand whether higher albedo materials can help cool the
microclimate of the courtyards or not. In the phase 1 and 2 simulations, the albedo of the
facades was 0.10, representing dark brick. In the new simulations, the albedo was changed
to 0.55 and 0.93, representing white marble and highly reflective plaster [70]. Light-coloured
plaster, materials and paint are used in the countries close to the equator with a high solar
radiation intensity. As a result, the increased albedo helps to reduce absorption of solar
radiation. This phase of the study investigated the potential of this strategy for cooler
climates.
Figure 12 shows how Tmrt change if a high albedo material replaces bricks on the facades of
the urban blocks 10*50 m2-EW. As can be seen, the model with plastered facades generally
has a higher air temperature. The hotter air in this courtyard at 16:00 h located mostly at the
eastern zone of the courtyard. Referring to Figure 13-b, the mean radiant temperature inside
the courtyards with high albedo facades during solar exposure is higher than in the
courtyards with low-albedo facades. Because of the higher albedo, more solar radiation is
reflected towards the central receptor point. Since a courtyard is a closed urban form, there
is a smaller probability to dissipate the solar reflections coming from buildings. In this way,
the 10*10 m2 courtyard has a higher increase (30 K at 15:00) in Tmrt than the bigger models.
The maximum increase in the 10*50 m2 EW model is +20°C (at 12:00), and +24°C (at 10:00)
for the 50*10 m2 NS model.
Figure 12: The effect of increased surface albedo from brick (0.10) to white marble (0.55) and plaster
(0.93) on mean radiant temperature of the 10*50 m2 EW model (left) and reflected solar radiation
(right).
4.4. Phase 4: The effect of water
Water installations such as fountains, canals, pools and ponds serve as heat buffers (with its
high specific heat capacity as a thermal mass) at urban, neighbourhood and building scales.
In the fourth phase of this study, the cooling effect of water (evaporation, heat buffering and
thermal mass) was considered for the three reference models. A water pool was embedded
on the ground inside the courtyard covering 65% of the area (the rest is left for walking
(pavement)). Figure 13-c presents Tmrt of the models with water (grey lines) and compares
this with the models without water from phase 1 (black lines).
The mean radiant temperature of the models with water is lower than of the models without
water. For the 10*50 m2-EW model, the maximum decrease is 18°C; and for the 50*10 m2-
NS model the maximum decrease is 21°C. Here it is worth mentioning that high-albedo
materials (e.g. the plaster used in phase 3) and water are both highly reflective substances.
The first showed a higher Tmrt in comparison with the phase 1 models while the latter showed
lower Tmrt. In case of a water pond, water on the ground reflects the high-altitude summer
sun back towards the sky and towards the receptor point. In contrast, the vertical high-albedo
materials (plasters on the facades) reflect the sun towards the ground of the courtyard and
also to the receptor point. Water has a high heat capacity as a result of which it does not
heat up as quickly as the concrete pavement does. Additionally, the evaporation of water
from the pond cools its surface and since the pond is close to the receptor point in the centre
of the courtyard, it has a strong effect on Tmrt.
4.5. Phase 5: The effect of vegetation
It was acknowledged in section 2 of this paper that the greening of urban spaces and the
installation of green roofs and porous pavements improves the air quality, reduces the
ambient air temperature and consequently lowers indoor air conditioning energy demands. In
the fifth phase of this study the ground of the courtyards and the roofs of the urban blocks
were covered with grass.
The first benefit of the grass is that it blocks the sun to the ground level of the courtyards.
Therefore, the soil absorbs less solar energy. Furthermore, the grass and soil cool the
ground surface and air layer above by evapotranspiration, an effect similar to the evaporative
cooling effect of the water ponds. In Figure 13-d, the differences between the green and
reference microclimates are shown (respectively grey and black lines). The largest difference
in mean radiant temperature is exhibited by the 50*10 m2 NS courtyard with a 17°C cooler
environment at 14:00 h due to the presence of grass.
5. Discussion
Alteration of a building’s geometry to receive or block sun is a common strategy in the early
stages of climate-sensitive design. This strategy was explored in the first phase of this study.
Different proportions of length to width in combination with four main orientations led to
varied microclimates. The N-S courtyard direction has the shortest duration of solar radiation
to penetrate into the courtyard, while the E-W orientation has the longest. The NW-SE
orientation receives sun in the early morning while the NE-SW orientation mainly in the
afternoon. Furthermore, by increasing the length of the courtyards only in the E-W direction,
the duration of solar radiation can be increased. In this way, among the models the 10*50 m2
EW courtyard model has the longest exposure to direct sun.
These results show that courtyards with N-S orientation are recommended for hot climates,
and E-W oriented courtyards are more favourable for colder regions. In addition, the NW-SE
direction is suitable when nights are cold and early morning sun is desired. In contrast, urban
spaces in cold climates that are used for afternoon activities (like recreational squares) are
more in accordance with NE-SW sun access.
Subsequently, the courtyard model with the longest duration of sun radiation (10*50 m2 EW)
was analysed and discussed to clarify the effects of different heat mitigation strategies: a)
use of a higher albedo material, b) use of a water pool, and c) use of vegetation. These three
strategies were analysed via their mean radiant temperature and air temperature.
As illustrated in Figure 12 (and Table 5), a high albedo of the facades leads to a higher mean
radiant temperature (maximum +20°C increase at 12:00 h); a water pool inside the courtyard
strongly reduces the mean radiant temperature (maximum -18°C at 15:00 h); vegetation also
strongly reduces the mean radiant temperature (maximum -17°C at 15:00 h). Considering
that the water pool covered 65% of the ground and the grass 100%, the cooling effect of
water seems to be more effective. Here it is worthy mentioning that several studies have
shown that increasing the albedo of facades leads to a cooler indoor environment; however;
this paper shows that outdoor environment performs different with higher albedos.
Figure 14 shows the air temperature of 10*50 m2 EW courtyard model at 16:00 h, which was
the hottest hour on the hottest reference day in the Netherlands. At this time, the sun
irradiates the model from the south-west, and thus the north-east side inside of the courtyard
is warmer, while the south-west side is cooler (Figure 14a). By increasing the albedo, the air
temperature in the courtyard increases mainly in the eastern part (which is more irradiated
than the western part at 16:00 h). Considering the outdoor space of the courtyard model
(surrounding the building), the air temperature in the regions outside the model (south and
west) are more increased because of using higher reflectance materials on facades (Figure
14b). In the third situation adding a water pool decreases the air temperature inside the
courtyard (Figure 14c); adding vegetation has a similar effect (Figure 14d). This is related to
the effects of water and vegetation on Tmrt: water more strongly affects Tmrt than vegetation
does. Furthermore, Figure 14 indicates the courtyard with water is cooler than the courtyard
with green at 16:00. This is mainly due to the higher specific heat capacity of water. This
allows water to absorb the sun during the day; in the afternoon, by releasing the heat, the
courtyard will be warmer than the courtyard with green (which is partly shaded by grass and
has absorbed less sun during the day).
Tmrt (°C) Ta (°C) RH (%)
Ref study 42 19 72
High albedo 53 20 71
Water pool 35 19 75
Greened 36 19 77
Table 5: The average mean radiant temperature (Tmrt), air temperature (Ta) and relative humidity (RH)
of the 10*50 m2 EW model.
Figure 13: Mean radiant temperature of the 10*50 m2 EW courtyard model comparing different heat
mitigation strategies.
Figure 14: Air temperature of the 10*50 m2 EW courtyard model in different phases of the study: a)
basic study, b) using high albedo facades, c) using water pool, and d) using grass.
6
13
20
27
34
41
48
55
62
69
76
83
90
05:0
0
06:0
0
07:0
0
08:0
0
09:0
0
10:0
0
11:0
0
12:0
0
13:0
0
14:0
0
15:0
0
16:0
0
17:0
0
18:0
0
19:0
0
20:0
0
21:0
0
22:0
0
23:0
0
00:0
0
01:0
0
Mea
n R
adia
nt
Tem
per
ature
(°C
) 10*50 Albedo
10*50
10*50 Green
10*50 Water
6. Conclusions
The urban heat island phenomenon and climate change have led and will further lead to a
temperature rise in urban spaces and cities. Therefore, there is a need for solutions to alter
microclimates and provide a more desirable environment for pedestrians. This paper
investigated the outdoor microclimate of different urban blocks during the hottest reference
day in the Netherlands. The courtyards were oriented in four main directions E-W, N-S, NE-
SW and NW-SE. Subsequently, they were widened from a symmetrical 10*10 m2 to 10*50 m2
in E-W and other directions.
The first phase of the study showed that on a summer’s day the E-W direction provides a
long duration of direct sun. In contrast, the N-S direction provides the shortest period of sun
radiation at the centre of a courtyard. Rotation of the models also showed that NW-SE-
oriented courtyards receive sun in the early morning while NE-SW-oriented courtyards
receive sun mostly in the afternoon.
Considering climate change and consequent global warming in 2050, three heat mitigation
strategies were investigated. Increasing the albedo of facades led to an extensive increase of
the mean radiant temperature (although it reduces indoor temperature). Using a water pool
inside the courtyard or covering the ground of the courtyard with vegetation significantly
reduced both the air temperature and mean radiant temperature. Finally, this research
suggests using water pool and green areas are the most effective heat mitigation strategies
for urban blocks in the Netherlands.
Appendix
a) E-W 10*10- 04h:32m 10*20- 07h:56m 10*30- 10h:00m 10*40- 11h:32m 10*50- 11h:44m
b) N-S 20*10- 04h:32m 30*10- 04h:32m 40*10- 04h:32m 50*10- 04h:32m
c) NE-SW 10*10- 03h:02m 10*20- 05h:13m 10*30- 05h:13m 10*40- 05h:13m 10*50- 05h:13h
d) NW-SE 20*10- 05h:13m 30*10- 05h:13m 40*10- 05h:13m 50*10- 05h:13m
Table 6: The duration of direct sun at the centre of the models in the reference study (Phase 1).
h = hour, and m = minute.
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